linux-kernel - Re: [PATCH] ARM: cpuidle: declare cpuidle_ops __read

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Date:	Wed, 10 Aug 2016 12:47:21 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jisheng Zhang <jszhang@...vell.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux@...linux.org.uk,
	daniel.lezcano@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: cpuidle: declare cpuidle_ops __read_mostly

On Wednesday, August 10, 2016 5:19:26 PM CEST Jisheng Zhang wrote:
> Dear Arnd,
> 
> On Wed, 10 Aug 2016 10:57:57 +0200 Arnd Bergmann wrote:
> 
> > On Wednesday, August 10, 2016 4:49:57 PM CEST Jisheng Zhang wrote:
> > > diff --git a/arch/arm/kernel/cpuidle.c b/arch/arm/kernel/cpuidle.c
> > > index 7dccc96..762e0929 100644
> > > --- a/arch/arm/kernel/cpuidle.c
> > > +++ b/arch/arm/kernel/cpuidle.c
> > > @@ -19,7 +19,7 @@ extern struct of_cpuidle_method __cpuidle_method_of_table[];
> > >  static const struct of_cpuidle_method __cpuidle_method_of_table_sentinel
> > >         __used __section(__cpuidle_method_of_table_end);
> > >  
> > > -static struct cpuidle_ops cpuidle_ops[NR_CPUS];
> > > +static struct cpuidle_ops cpuidle_ops[NR_CPUS] __read_mostly;  
> > 
> > Should this perhaps be percpu data instead?
> > 
> 
> Per my understanding, percpu is used for those vars with normal read/write
> frequency, while the cpuidle_ops is read mostly, so IMHO, __read_mostly
> is suitable, what do you think?

You are right, __read_mostly is better than the normal .data section here,
but percpu is also better than .data because it saves a little memory
on machines that have few present CPUs than CONFIG_NR_CPUS.

So both have their advantages, we just need to pick a preference.

Actually __ro_after_init would be even better than __read_mostly here
I think, as this is only updated in an __init function. I guess
using that would have the added security advantage of preventing
an attacker from writing to the function pointers when they
find a way to overflow an access in the percpu data section.

	Arnd